Support and Movement

Bone is the toughest living structure in the body, and it manages to be both the toughest thing you own and a living tissue because of its matrix. That splits into an organic and an inorganic part: collagen resists being pulled, and hydroxyapatite, mostly calcium phosphate, is the mineral deposited over it during calcification or ossification, with 10 to 20% water. The inorganic matrix constitutes about 65% of the bone's dry weight. Collagen alone gives a framework that pulls apart; hydroxyapatite alone gives a chalk stick. Osteology, the study of bones, shares the root.

A long bone has three regions. The epiphyses at each end are cancellous or spongy bone built of weight-bearing struts called trabeculae, and hold red bone marrow, which is about 20% of the total mass of the skeleton. The shaft, the diaphysis, carries cortical bone, almost 80% of the skeletal structure, built from osteons: cylinders of concentric lamellae with a haversian canal at the centre carrying blood and nerves. Outside everything is the periosteum, which anchors tendons and admits blood and lymphatic vessels; inside the medullary cavity is the endosteum, a thin layer of osteogenic progenitor cells giving rise to osteoblasts and chondroblasts, which is why it matters in fracture repair.

The three cell names carry the whole of bone biology, and the suffix is the verb. Osteoblasts secrete matrix around themselves, producing spongy bone that later becomes compact bone. Osteoclasts break the material down, and they are described as multinucleated and phagocytic, resorbing mineralised matrix during remodelling. Osteocytes are the mature cells, sealed into lacunae by the matrix they laid down, keeping the tissue healthy by secreting enzymes and directing osteoblasts to damage. While some sources may credit osteocytes with phagocytosing bony matrix, the accepted answer for dissolving bone is osteoclast.